1. Field of the Invention
The present invention relates to organic electroluminescence (EL) display devices, and more particularly, to an organic EL display device with improved light bleeding efficiency of light emitted from an organic film.
2. Description of the Related Art
Organic EL display devices are spontaneous light-emitting display devices that emit light by electrically exciting a fluorescent organic compound. Typically, organic EL display devices may operate at a low voltage, be formed compactly to form a thin device, and provide a wide viewing angle and a high response speed. Organic EL display devices are the focus of considerable attention because they may be used as next-generation display devices that are free of the problems of liquid crystal display devices.
In such organic EL display devices, a predetermined pattern of organic films is formed on glass or a transparent insulative substrate, and electrode layers are formed on the top and bottom surfaces of the organic film pattern. The organic films may be made of various organic compounds such as copper phthalocyanine (CuPc), N,N′-Di (naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB), and tris-8-hydroxyquinoline aluminum (Alq3).
In organic EL display devices having the above-described structure, as positive and negative voltages are applied to electrodes, holes migrate from electrodes to which the positive voltage is applied to a luminescent layer via a hole transport layer and electrons migrate from electrodes to which the negative voltage is applied to the luminescent layer via the hole transport layer. In the luminescent layer, the holes and the electrons re-unite to generate excitons. As the excitons de-excite, fluorescent molecules in the luminescent layer emit light, thus forming an image.
The light efficiency of organic EL display devices driven as described above includes internal efficiency and external efficiency (or light bleeding efficiency). Internal efficiency relates to the efficiency of photoelectric conversion of an organic luminescent material, and external efficiency depends on the refractive indices of the layers that constitute the organic EL display device. In other words, when light is emitted from the organic films at a critical angle or greater, the emitted light is reflected at the interface between the substrate and the electrode layers or between the organic films and the electrode layers. Thus, the emitted light is prevented from being bled off.
In a conventional organic EL display device, as shown in FIG. 1, when light emitted from an organic film 13, which is protected by a metal cathode layer 14, is transmitted through an interface between indium tin oxide (ITO) electrodes 12 and a transparent glass substrate 11, to the transparent glass substrate 11, the light transmittance T is given by: T=1/2(nglass/nITO)2, wherein nglass denotes the refractive index of glass and nITO denotes the refractive index of ITO.
Based on the above equation, the light bleeding efficiency for individual colors of a conventional organic EL display device is shown in the following table.
BlueRedorganic filmorganic filmGreen organic filmWavelength (nm)460620530Refractive index (n)2.011.761.93of ITO electrodesRefractive index (n)1.5251.5151.52of glass substrateLight bleeding29%37%34%efficiency
As shown in the above table, 60% or more of the light generated in an organic EL display device is extinguished within the device due to the difference between the refractive indices of the ITO electrodes and the glass substrate.
Japanese Patent Publication No. hei 11-283751 discloses an organic EL display device having a structure in which a diffraction grating or a zone plate is formed on a substrate to diffract light guided by ITO electrodes and by an interface between organic films and the ITO electrodes, thus reducing light loss. Since this organic EL display device requires a wavy substrate, a wavy fine electrode pattern layer, or a special diffraction grating, its manufacturing process is complicated. Also, when an organic layer is formed on the wavy surface or the diffraction grating, the surface illumination of the organic layer increases. Thus, the leakage current of the organic EL display device increases and durability and reliability decrease.
An example of a conventional organic EL display device which prevents degradation of light bleeding efficiency is disclosed in Japanese Patent Publication No. sho 63-172691. The disclosed organic EL display device includes a condensed substrate such as a substrate on which a protrusion lens is installed. However, since pixels depending on the luminescence of an organic film are very small, it is difficult to install the protrusion lens for condensing on the substrate.
Japanese Patent Publication No. hei 1-220394 discloses an organic EL display device in which lower electrodes, an insulating layer, a luminescent layer, and upper electrodes are formed on a substrate, and a mirror that reflects light is installed on a single face of the luminescent layer. In this organic EL display device, since the luminescent layer is very thin, it is very difficult to install a reflective mirror on one side of the luminescent layer.
However, Japanese Patent Publication No. hei 9-171892 discloses an organic EL display device in which anodes and an anode interface layer are formed on a glass substrate having a lens-shaped structure installed thereon, and a hole transport layer, a luminescent layer, an electron transport layer, cathodes, and an anode protective layer are formed on the resulting glass substrate. In this organic EL display device, since reflected light bleeds toward the glass substrate, the bleeding efficiency increases, but images become blurred.
Japanese Patent Publication No. hei 10-12382 discloses an organic EL display device that has a light guide formed between front electrodes of an organic luminescent device and a group of small lenses incorporated into the front surface of the light guide, such that emitted light is guided to the front electrodes, and light bleeds evenly.
U.S. Patent Publication No. 2001/0019242A1 also discloses an organic EL display device and a method of manufacturing the same.